TOPS Image Quality and Processor Verification Study - Final Report

Kurzfassung

This is the final report summarizing all the activities performed by the German Aerospace Center (DLR), Microwaves and Radar Institute (HR), within the framework of the “TOPS Image Quality and Processor Verification Study” under ESA/ESTEC contract number 22243/09/NL/JA. The main mode of operation for ESA’s Sentinel-1 SAR system is the so-called TOPS (Terrain Observation with Progressive Scan) mode which provides large swath observations with enhanced image performance compared to the traditional ScanSAR mode. In a precursor ESA study performed by DLR-HR (C20679/07/NL/CB “Sentinel-1 TOPS Imaging Mode Demonstration with TerraSAR-X”) the feasibility of this new mode has first been demonstrated evaluating experimental TOPS acquisitions of the German TerraSAR-X satellite TSX. The purpose of the present study is to investigate in more detail specific properties of the new TOPS mode to evaluate whether the ambitious requirements of the S-1 products can be ensured by the present TOPS system design and to identify additional compensation strategies, wherever required. For this purpose several experimental TerraSAR-X acquisitions were scheduled manually and investigated subsequently. The study is divided into four major parts: Chapter 1 investigates two technical questions relating to the TOPS mode of operation. First the problem of Doppler centroid variations at high azimuth steering angles is evaluated as a function of swath width and terrain topography. For this purpose several TSX stripmap acquisitions where scheduled with high azimuth steering angles of 1 deg. The question to be addressed was to identify any possible precautions to be taken in the processor for optimum focussing quality. The second problem relates to the residual scalloping correction which is induced by the electronic antenna steering during acquisition of each TOPS burst. Dedicated TSX TOPS data takes were commanded over Amazonian rainforest and were evaluated for the purpose of defining suitable scalloping correction methods. The second chapter relates to the investigation of particular signal properties of the TOPS mode, specifically with respect to interferometric applications. The azimuth variant Doppler centroid within each burst leads to increased coregistration requirements, which have been quantified and verified using interferometric data sets acquired by TSX in TOPS mode. Time series evaluations of TSX descending Mexico City acquisitions were used to assess and confirm for the first time the suitability of TOPS data for subsidence mapping together with a characterisation of coherent scatterers (CS) for the TOPS mode. The phenomenon of flashing fields has been investigated for the TOPS burst overlay areas using a second time series acquired over Flevoland. Chapter 3 is dedicated to the validation of the operational TOPS processing algorithm. Simulated Sentinel-1 and TerraSAR-X raw data scenarios were defined resulting in the commanding and acquisition of real TerraSAR-X raw data. The processing results of both processors under investigation, i.e. the Sentinel-1 prototype TOPS (SPT) processor and the experimental TerraSAR-X TOPS processor (ETT) were compared. Finally, chapter 4 investigates the innovative Flexible-Dynamic Block Adaptive Quantization (FDBAQ) algorithm for the first time with real spaceborne SAR data. FDBAQ is implemented operationally onboard S-1 to ensure improved data quantization in terms of Signal to Quantization Noise (SQNR) and more efficient data quantization. Dedicated data acquisitions were performed by TSX in stripmap mode, switching off the default onboard BAQ compression. FDBAQ compression results were assessed and compared to the less performing Entropy Coded BAQ (ECBAQ). For each of the investigated topics conclusions are drawn and recommendations for Sentinel-1 are given were appropriate.